Ninebark (Physocarpus opulifolius, family Rosaceae) has become a popular deciduous woody ornamental in Ohio landscapes. The native deciduous shrub is valued for its horticultural appeal and durability over a wide range of environmental conditions.
However, some ninebark selections can suffer heavy infections by the Powdery Mildew fungus, Podosphaera physocarpi (= Podosphaera aphanis var. physocarpi). The fungus behaves nothing like the more mild-mannered surface-dwelling forms such as Erysiphe syringae, the powdery mildew fungus that is common on common lilacs (Syringa vulgaris).
Powdery mildew on ninebark can be highly aggressive, producing gnarled, twisted leaves adorned with a thick coating of snow-white mycelia. New leaves are highly susceptible, with heavy infections causing the leaves to become stunted and deformed. Eventually, the leaves dehydrate and die.
Ninebark is appreciated for its beautiful, delicate flowers visited by plant pollinators. However, their beauty and usefulness to pollinators are marred after becoming encased in fungal growth.
Infections progress into the stems, causing dieback. Heavy infections not only limit the plant’s landscape value but can also cause plants to slowly fade away.
The relationship between ninebark and its fungal nemesis provides a good case study on using the plant Disease Triangle. The Triangle has its roots in the 1960s; however, it continues to serve as an effective tool for teaching plant disease development and management. It’s both simple as a graphical concept and elegant in its representation of the paradigms in plant pathology.
The Disease Triangle illustrates the three conditions that must be met for a plant disease to develop. All three must be present at the same time. The power of the triangle is that removing only one of these conditions will prevent disease development.
The three conditions are:
- HOST: A host plant must be susceptible to infection.
- ENVIRONMENT: Environmental conditions must be present that support infection and disease development.
- PATHOGEN: A virulent pathogen must be present.
Applying the Disease Triangle
The Host
Several factors can influence a plant's susceptibility to infection, including plant stress. However, published research shows that plant genetics is a primary factor, with some cultivars being much more susceptible.
For example, I was crushed when my deep purple-leaved Diabolo® (P. opulifolius ‘Monlo’) planted in a high-profile location in 2006 became hammered with powdery mildew, starting in 2007. Heavy infections began converting my ‘Monlo’ into ‘Snowlo.’
My ninebark suffered from heavy infections for three consecutive years. Instead of increasing in height and width, it was shrinking owing to stem dieback and continuous pruning to remove infectious tissue (“sanitation pruning”).
Around the same time, I visited native ninebark growing in Vinton Furnace State Forest in Vinton County, Ohio. The plants were huge and growing at the edge of a forest. The wild ninebark showed no signs of powdery mildew infections.
Applying the Disease Triangle to my observation, perhaps the pathogen wasn’t present, or the environment had failed to support infection and disease development. Of course, given that the native ninebark and the native powdery mildew fungus co-evolved, perhaps genetic resistance forged by natural selection was providing protection. This would imply that resistance was lost in my ninebark through unnatural selection.
The Environment
In general, powdery mildews thrive where moisture is freely available for extended periods. But not too much water; the fungus can be washed from plants by heavy rainfall.
My ninebark was planted in a location with minimal sunlight and limited airflow. It was backed by a wall and flanked by shrub junipers (Juniperus overuses var. ubiquitous). It was also shaded by a large Callery pear (Pyrus callerinvasivus var. obnoxius). The ninebark foliage remained wet for extended periods after rainfall or heavy morning dew, creating perfect conditions for powdery mildew infections.
In a last-ditch Hail Mary project to save my ninebark from the ravages of powdery mildew, I moved it to my backyard. The new site is in full sun, with an east-west exposure, and airflow from the west is unimpeded by nearby structures or vegetation.
The effect was dramatic. The ninebark remains in my landscape, and I’ve never found a single site of powdery mildew infection in the 16 years that it’s been in the new location. The image below shows the current condition after a heavy rejuvenation pruning this spring.
My personal experience with the power of the environment has been supported by other observations. The image below shows Diabolo® in a park in Cincinnati. I could only find powdery mildew infections on the portion of the plants that was shaded by a tree and where air flow was limited.
The images below show Diabolo® in a park in Dayton. The environmental conditions conducive to infection are provided by the large oaks shading the ninebark plants. Note the size differences between the plants in the shade and those in full sun. The smaller plants are the result of stem dieback and pruning in response to powdery mildew infections.
The images below show Diabolo® in another park in Cincinnati. The plants are positioned in the shade next to a misting structure to cool visitors. Taken together, the environmental conditions provide a perfect recipe for powdery mildew.
The Pathogen
Powdery mildew fungi are so named because of the white, powder-like growth that covers plant surfaces. Their thread-like hypha are microscopic, but when multitudes of hyphae assemble, they form macroscopic mycelia (mycelium = singular). It’s analogous to forming a rope from individual threads. It’s the white mycelia that we see covering leaf surfaces.
However, powdery mildew of ninebark doesn’t follow the rules. It doesn’t just live on leaf surfaces. The fungus also appears to penetrate leaves producing raised, wart-like structures that harbor the fungus. It also penetrates stems producing stem dieback symptoms.
This means that throwing powdery mildew off the disease train by sanitation pruning or spraying fungicides is problematic. Neither is effective if applied after the disease train has left the station.
As noted above, I attempted to manage powdery mildew on my ninebark by sanitation pruning to remove the pathogen. However, I was never able to get ahead of the fungus.
I did not apply a fungicide, and the plants in the parks shown in the photographs above had not been sprayed. However, fungicide applications are commonly cited as a method for removing the pathogen from the Disease Triangle.
A research paper published in 2021 (Baysal-Gurel and Bika) revealed that the following fungicides are effective in suppressing powdery mildew on ninebark: azoxystrobin + benzovindiflupyr (Mural), and chlorothalonil + propiconazole (Concert II). The Pacific Northwest Plant Disease Management Handbook (2026) adds to the list by recommending trifoxystrobin + triadimefon (Armada 50 WP), pyriofenone (Seido), and myclobutanil (Myclobutanil 20 EW T&O).
Of course, as with using any pesticide, applicators must read and follow label directions. This is particularly important with relying on fungicides to suppress disease development. Application timing and dosage are critical. It’s also important to recognize that the continuous use of fungicides with the same Fungicide Resistance Action Committee (FRAC) code risks selecting a resistant strain of the fungus.
The Bottom Line
The power of the Triangle rests with the proposal that removing only one of the three conditions required for disease development will prevent disease development. However, while the shape of the Triangle implies that the three conditions are equal, in reality, some conditions are more equal than others.
For example, it should be obvious at this point that Diabolo® is susceptible to powdery mildew. By far, the most effective and long-lasting powdery mildew management strategy would be to select a genetically resistant ninebark.
The Pacific Northwest Plant Disease Management Handbook (2026) notes that ‘Nanus’ did not develop infections during a two-year study in Connecticut. While some infections may occur on ‘Seward’ [Summer Wine™] and ‘Luteus,’ they are still considered resistant.
Although Diabolo® is vulnerable to powdery mildew, its susceptibility can be suppressed or reinforced by the environment, as illustrated in the examples above. I suppressed powdery mildew by using the environment. However, I’m well aware that my ninebark is not genetically resistant. I could experience a reversal of fortune with a change in environmental conditions that are beyond my control.
The reason I did not make fungicide applications is that trying to spray away a highly virulent pathogen on a highly susceptible host is a highly risky endeavor. It's all about timing, with a missed application undercutting the effort by opening the door for disease development.
Sanitation pruning also presents risks. I’m not suggesting that pruning away infectious plant tissue isn’t effective. However, the efficacy depends on recognizing and removing the parts of the plants harboring the pathogen, which is based on seeing signs and symptoms. What if both are hidden from view?
The bottom line is that it’s important to recognize that all sides of the Disease Triangle are not equal. The most effective approach is to devise a “stepwise” management strategy starting with host resistance, followed by addressing environmental issues. Fungicides and pruning are powerful tools, but problematic, meaning they should not be the first step, but perhaps the last.
A Final Note
I came across an unusual lady beetle belonging to the genus Psyllobora (family Coccinellidae) while inspecting ninebark plants infected with powdery mildew. Unlike most lady beetles that eat meat, members of this genus dine on fungi, especially powdery mildews.
Although their food preference may make them a candidate for biological control of powdery mildews, the possibility has attracted a limited number of scientific investigations. Regardless, the existence of powdery mildew-eating lady beetles illustrates the wide diversity in insect lifestyles.
Selected References
Baysal-Gurel, F., & Bika, R. (2021). Management of powdery mildew on ninebark using sanitizers, biorational products, and fungicides. HortScience, 56(5), 532-537.
Baysal-Gurel, F., Simmons, T., Avin, F. A., Bika, R., & Jennings, C. (2021). First report of powdery mildew on Physocarpus opulifolius caused by Podosphaera physocarpi in Tennessee. Plant disease, 105(1), 216.
https://apsjournals.apsnet.org/doi/full/10.1094/PDIS-06-20-1286-PDN
Francl, L.J. (2001) The disease triangle: a plant pathological paradigm revisited. The Plant Health Instructor, DOI: 10.1094/PHI-T-2001-0517-01.
https://www.apsnet.org/edcenter/learningPP/Pages/DiseaseTriangle.aspx
Hawke, R. (2021, December). Ninebark: A durable shrub for everyone (Plant Trial Results No. FG202). Chicago Botanic Garden. https://www.chicagobotanic.org/sites/default/files/2023-12/FG202_Ninebark-A%20Durable%20Shrub%20for%20Everyone.pdf
Ninebark (Physocarpus spp.)-Powdery mildew. (2026). In Pacific Northwest Plant Disease Management Handbook. Oregon State University, Washington State University, & University of Idaho. https://pnwhandbooks.org/plantdisease/host-disease/ninebark-physocarpus-spp-powdery-mildew
Scholthof, K. B. G. (2007). The disease triangle: pathogens, the environment and society. Nature Reviews Microbiology, 5(2), 152-156.
https://www.nature.com/articles/nrmicro1596
Stevenson, J. A. (1960). Plant Pathology. An advanced treatise. JG Horsfall and AE Dimond, Eds. vol. 1. The Diseased Plant. Academic Press, New York, 1959. xiv+ 674 pp. Illus. Science, 131(3410), 1368-1368.
https://www.science.org/doi/abs/10.1126/science.131.3410.1368.a
Sutherland, A. M., & Parrella, M. P. (2009). Biology and co-occurrence of Psyllobora vigintimaculata taedata (Coleoptera: Coccinellidae) and powdery mildews in an urban landscape of California. Annals of the Entomological Society of America, 102(3), 484-491.
https://academic.oup.com/aesa/article/102/3/484/8626
